Small tape drives for writing and reading data stored on tape in mini-cartridges are commonly used in computer systems. The 1/4-inch tape size is one popular size. U.S. Pat. No. 4,863,114-Moeller, et al. shows a mini-cartridge in which magnetic tape stores computer data. U.S. Pat. No. 3,526,371-Blackie et al; U.S. Pat. No.3,924,823-Cohen, et al; U.S. Pat. No. 4,647,994-Irwin, et al; and U.S. Pat. No. 4,984,111-Rudi are examples of drives utilizing this type of data cartridge.
Quarter-Inch Cartridge Drive Standards Inc, 311 East Carillo Street, Santa Barbara, Calif. 93101 publishes development standards adopted by several manufacturers for tape drives. These standards describe an 80/120 megabyte, 28-track, 14,700 bpi (579 bmm) MFM-encoded flexible disk controller compatible recording format using a 1/4-inch mini-data cartridge. Published standards included "FLEXIBLE-DISK-CONTROLLER-COMPATIBLE RECORDING FORMAT FOR INFORMATION INTERCHANGE", QIC-80 Revision D, Dec. 6, 1989, "COMMON COMMAND SET INTERFACE SPECIFICATION FOR FLEXIBLE DISK CONTROLLER BASED MINI-CARTRIDGE TAPE DRIVES", QIC-117, Revision B, Dec. 6, 1989; and "SERIAL RECORDED MAGNETIC TAPE MINICARTRIDGE FOR INFORMATION INTERCHANGE," QIC-3020. These published standards are incorporated herein by reference. Companies which make tape drives for reading and writing tapes generally to these standards include: Mountain Network Solution, 240 East Hacienda Avenue, Campbell, Calif. 95008; Wangtek Corp., 41 Morehand Road, Seme Valley, Calif. 92605; Archive Corp., 1650 Sanflower Avenue, Costamesa, Calif. 92626; CMS Enhancements, 1372 Valencia Avenue, Fustin, Calif. 92680; and Iomega Corp., San Diego, Calif., which is the assignee of the present invention.
Similar tape drives are available which write tapes in a format which is not compatible with the QIC standards. One example of such a drive is that manufactured by Irwin Magnetics, Inc. U.S. Pat. Nos. 4,646,175; 4,586,094 and 4,498,129-Chambors, et al. describe the tape drive and format of tapes written on these drives.
Regardless of whether the drives comply with QIC standards, conventional tape recording and reproducing systems for use as computer data storage devices are required to provide high data transfer rates and high linear information density. To satisfy these requirements, conventional tape systems typically employ methods of recording known as linear recording, in which the tracks of lie parallel to each other and to the edge of the tape, or helical scan recording, in which the tracks of data lie parallel to each other but diagonal to the edge of the tape. Tracks of data typically comprise discrete, magnetized bits of information that are produced on the magnetic tape in an elongate shape. A series of the bits are recorded along the length of the tape to produce the track.
Tape track density is limited by lateral tape motion, which is the random and unavoidable tendency for a tape randomly to drift in a direction normal to the longitudinal direction of tape motion. During the writing process, lateral tape motion causes track position to deviate from the parallel to the edge of the tape. During the reading process, lateral tape motion causes mis-registration of the read head over the track being read, which may result in read data error. Although the mis-registration typically manifests only during the reading process, the lateral tape motion occurring both during the reading and writing processes causes the read head position error.
Moreover, even without lateral tape motion, the physical space required for each track is a limiting factor on overall tape density. One limiting factor is head size. Unfortunately, diminishing the size of the heads, which has been the industry trend, typically results in protracted development cycles and diminished competitive advantage at the time of market introduction. Tape track densities are also limited by crosstalk, which occurs when reading is interfered with by data of adjacent tracks. Crosstalk is exacerbated by error in head gap alignments and by lateral tape motion. Techniques to minimize the crosstalk-related problems include leaving guard bands between tracks, or using wider write head gaps or wider tracks. These techniques, however, limit track densities.
Although the linear recording method offers higher data transfer rates, it is desirable to obtain higher data densities while retaining the advantages of this method. Various methods of increasing tape track densities have therefore been pursued. A well-known method of recording known as azimuth recording has been used in helical scan recording systems, and has recently been applied in linear tape systems to increase the track density of these systems. Azimuth recording results in a recorded track pattern in which the magnetization directions of adjacent data tracks lie at different azimuth angles to each other. This method greatly reduces crosstalk from adjacent tracks, allowing tracks to be placed closer together. The need for guard band spaces between tracks, wide write heads or narrow read heads is thus reduced or eliminated. U.S. Pat. No. 5,293,281, entitled "Method of Reading and Writing Data Transitions on Side-By-Side Tracks on Magnetic Media," (Behr), which is incorporated herein by reference in its entirety, employs azimuth recording.
Another technique is to record servo information onto the tape at various locations along its length or on separate levels. U.S. Pat. No. 5,523,904, entitled "Linear Tape Write Servo Using Embedded Azimuth Servo Blocks," (Saliba), which is incorporated herein by reference in its entirety, and the Behr U.S. Pat. No. 5,293,281 provide such a servo system. Unfortunately, such servo systems utilize tape that may otherwise be available for storing data, limits track density, and add cost and complexity to the drive. Additionally, servo written cartridges require development of expensive servo writing equipment used for the manufacture of said cartridges (servo writers). The combination of added drive and cartridges costs as well as extended development cycles only work against the two prime requirements (affordability and timeliness) for a successfull product introduction. More specifically, the additional drive functions needed to accommodate recorded track following schemes include, but are not limited to: linear head actuator, control electronics, demodulator circuit, additional microprocessor bandwidth, larger program memory (ROM and RAM), and substantially more complex firmware. There is, therefore, a need for techniques to increase performance and capacity without using closed loop servo schemes.
The Behr patent further describes a technique for overlapping a portion of a first track by a subsequent track to produce a narrowed first track. However, the Behr patent describes data disposed only in an azimuthal orientation. Such azimuth recording has the advantage of diminishing crosstalk because the head reading a certain track will be angularly offset from the adjacent track. However, in drives described herein, the use of a single inexpensive, readily available monolithic read-write head precludes the use of the Behr technique that requires multiple alternate azimuth heads or a single head with complex titling mechanics and control electronics. Moreover, aligning heads during assembly to the precise azimuth angles typically adds cost to the azimuth recording system. Furthermore, such systems lack compatibility with other tape systems.
Therefore it is desirable to provide techniques for increasing track density of a linear tape while diminishing the risk of data error due to crosstalk to increase compatibility, and to reduce cost and complexity of the drive. The foregoing and other objects, features and advantages of the invention will become evident hereinafter.